In recent years, average temperatures in the county seat of Belvidere have ranged from a low of 11 °F (−12 °C) in January to a high of 85 °F (29 °C) in July, although a record low of −29 °F (−34 °C) was recorded in January 1979 and a record high of 109 °F (43 °C) was recorded in July 1936. Average monthly precipitation ranged from 1.29 inches (33 mm) in February to 4.56 inches (116 mm) in June.[6]

As of the 2010 United States Census, there were 54,165 people, 18,505 households, and 14,273 families residing in the county.[12] The population density was 193.0 inhabitants per square mile (74.5/km2). There were 19,970 housing units at an average density of 71.1 per square mile (27.5/km2).[5] The racial makeup of the county was 84.4% white, 2.0% black or African American, 1.3% Asian, 0.4% American Indian, 9.3% from other races, and 2.6% from two or more races. Those of Hispanic or Latino origin made up 20.2% of the population.[12] In terms of ancestry, 25.6% were German, 13.7% were Irish, 9.2% were English, 8.6% were American, 7.7% were Italian, 7.1% were Swedish, and 5.0% were Polish.[13]

Of the 18,505 households, 41.5% had children under the age of 18 living with them, 61.6% were married couples living together, 10.2% had a female householder with no husband present, 22.9% were non-families, and 18.9% of all households were made up of individuals. The average household size was 2.91 and the average family size was 3.32. The median age was 36.8 years.[12]

The median income for a household in the county was $61,210 and the median income for a family was $69,380. Males had a median income of $53,581 versus $34,651 for females. The per capita income for the county was $26,105. About 7.9% of families and 10.4% of the population were below the poverty line, including 15.3% of those under age 18 and 5.0% of those age 65 or over.[14]

As a historic Yankee settlement, Boone County in its early years was a major base for the Free Soil Party, being one of nine Illinois counties to vote for Martin van Buren in 1848. Its Free Soil affinities meant Boone became one of the first strongholds of the Republican Party and remained overwhelmingly Republican for the following century, although it did vote for ProgressiveTheodore Roosevelt in 1912 when the Republican Party was severely split. Between at least 1892 and 1928 no Democratic Presidential candidate ever managed twenty percent of the county’s vote, and in the century up to 1960 no Democrat reached thirty percent – a degree of GOP loyalty comparable to such famous bastions as Owsley County in Kentucky, Grant County in West Virginia, or Avery County in North Carolina. Even Barry Goldwater, who alienated the Yankee Northeast so much as to lose all but one county there, still won Boone County by 15.6 percentage points, and between 1968 and 1988 no Democrat did better than Jimmy Carter’s 40.2 percent.

The shift of the Republican Party towards an expanded Southern and Western base, and particularly its growing strength with social conservatives, alienated the Yankee North during the 1990s and 2000s, but Boone County remained in Republican hands. The candidacy of Ross Perot in 1992 and 1996 caused George H. W. Bush and Bob Dole both to win Boone County with mere pluralities against Bill Clinton, and in 2008, Illinois resident Barack Obama became the only Democrat to carry the county since James K. Polk in 1844. The 2010s have seen a reversal of this Democratic trend due to concern over employment declines in the “Rust Belt“, so that Donald Trump’s 2016 showing was the best by a Republican since 1988.